Repair private virtual member output

Metadata can contain private virtual members that C# cannot declare directly. Adjusting the emitted accessibility in the decompiler keeps such members compilable without pushing the policy into the general AST builder.

Assisted-by: OpenCode:openai/gpt-5.5:OpenCode

ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs

@@ -117,6 +117,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await Run();
 		}
 
+		[Test]
+		public async Task Issue3136()
+		{
+			await Run();
+		}
+
 		[Test]
 		public async Task Issue1918()
 		{

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue3136.cs

@@ -0,0 +1,16 @@
+using System.Runtime.CompilerServices;
+
+public class MyClass
+{
+	private object _MyValue;
+
+	protected virtual object MyValue {
+		get {
+			return _MyValue;
+		}
+		[MethodImpl(MethodImplOptions.Synchronized)]
+		set {
+			_MyValue = value;
+		}
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue3136.il

@@ -0,0 +1,35 @@
+.class public auto ansi beforefieldinit MyClass extends [mscorlib]System.Object
+{
+	.field private object _MyValue
+
+	.method public hidebysig specialname rtspecialname instance void .ctor () cil managed
+	{
+		.maxstack 8
+		ldarg.0
+		call instance void [mscorlib]System.Object::.ctor()
+		ret
+	} // end of method MyClass::.ctor
+
+	.method private specialname newslot strict virtual instance object get_MyValue () cil managed
+	{
+		.maxstack 8
+		ldarg.0
+		ldfld object MyClass::_MyValue
+		ret
+	} // end of method MyClass::get_MyValue
+
+	.method private specialname newslot strict virtual instance void set_MyValue (object 'value') cil managed synchronized
+	{
+		.maxstack 8
+		ldarg.0
+		ldarg.1
+		stfld object MyClass::_MyValue
+		ret
+	} // end of method MyClass::set_MyValue
+
+	.property instance object MyValue()
+	{
+		.get instance object MyClass::get_MyValue()
+		.set instance void MyClass::set_MyValue(object)
+	} // end of property MyClass::MyValue
+}

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -2245,6 +2245,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					// without this the decompiled program does not compile (CS5001).
 					methodDecl.Name = "Main";
 				}
+				FixPrivateVirtualMemberModifiers(methodDecl, method);
 				FixParameterNames(methodDecl);
 				var methodDefinition = metadata.GetMethodDefinition((MethodDefinitionHandle)method.MetadataToken);
 				if (!settings.LocalFunctions && LocalFunctionDecompiler.LocalFunctionNeedsAccessibilityChange(method.ParentModule!.MetadataFile, (MethodDefinitionHandle)method.MetadataToken))
@@ -2824,6 +2825,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					int lastDot = property.Name.LastIndexOf('.');
 					propertyDecl.Name = property.Name.Substring(lastDot + 1);
 				}
+				FixPrivateVirtualMemberModifiers(propertyDecl, property);
 				FixParameterNames(propertyDecl);
 				Accessor? getter, setter;
 				if (propertyDecl is PropertyDeclaration)
@@ -2905,6 +2907,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				{
 					eventDecl.Name = ev.Name.Substring(lastDot + 1);
 				}
+				FixPrivateVirtualMemberModifiers(eventDecl, ev);
 				if (isAutomaticEvent)
 				{
 					AutoEventDecompiler.AddFieldLikeEventAttributes((EventDeclaration)eventDecl, typeSystemAstBuilder, ev, backingField!);
@@ -2941,6 +2944,16 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
+		static void FixPrivateVirtualMemberModifiers(EntityDeclaration declaration, IMember member)
+		{
+			if (member.Accessibility != Accessibility.Private || member.DeclaringType.Kind == TypeKind.Interface)
+				return;
+			if (!member.IsVirtual && !member.IsAbstract && !member.IsOverride)
+				return;
+			declaration.Modifiers &= ~Modifiers.Private;
+			declaration.Modifiers |= Modifiers.Protected;
+		}
+
 		#region Sequence Points
 		/// <summary>
 		/// Creates sequence points for the given syntax tree.